Transmission system



L. J. LESH TRANSMISSION SYSTEM Filed Jan. 23, 1930 May 29, 1934.

n'fi or Laurens! .1 Leah Patented May '29, 1934 UNITEB STATES PATENT GFFIE TRANSMISSION SYSTEM ware Application January 23 10 Claims.

The present invention relates in general to transmission systems and more particularly to carrier current telephone and telegraph systems. The main object of the invention is the provision of a novel form of oscillator especially suitable for use in systems of the above type.

According to one feature of the invention the oscillator comprises two neon tube oscillating circuits having a common branch which tends to synchronize the two circuits'so' that they operate at the same frequency.

Another feature of the invention relates to the novel method of modulating the output of said oscillator by shifting the phase relation between the currents of the two sections of the oscillator and at the same time keeping them synchronized at the same frequency.

Other features of the invention willbe apparent from the following detailed specification when read in connection with the accompanying drawing comprising Figs. 1 to 3, inclusive, in which Fig. 1 shows one station of a carrier current telephone system according to the present invention, 7

Fig. 2 is a modification of the oscillator of Fig. 1, using a differential condenser microphone instead of the usual telephone transmitter, and

Fig. 3 shows a carrier current telegraph system embodying the present invention.

Referring to Fig. 1 of the drawing, one station of a carrier current telephone system is shown comprising a transmitting and receiving circuit coupled to the line comprising conductors 1 and 2, by means of the usual balanced coil arrangement H and the artificial line 33; The transmitter consists of a neon tube oscillator comprising two oscillating tubes 5 and 6 in push-pull relationship. The two circuits are coupled by the resistance 12 and coil 13. The telephone transmitter in series with a battery is inductively coupled to the common branch of the oscillator. The output of the oscillator is amplified bythe two resistance coupled amplifiers-16 and 23. The receiving circuit consists of a rectifying tube 36 and a telephone receiver 41.

In the modification oi the oscillator shown in Fig. 2, the telephone transmitter 15 and battery 46 of Fig. 1 are replaced by a diiierential condenser microphone, the vibrating element of which is connected to the common branch of the oscillator and the two condenser elements of which are shunted around the two neon tubes.

In Fig. 3 of the drawing is shown one station of a carrier current telegraph system. The

transmitting and receiving circuits are coupled to 1930, Serial No. 422,913

the telegraph line, comprising conductors 101 and 102, in the same manner as in Fig. 1. The

telephone transmitter of Fig. 1 is replaced by a neon tube oscillator which may be intermittently coupled to the common branch of the carrier current oscillator, in accordance with the telegraph signals, by means of the key 150. In the receiving circuit, the telephone receiver is replaced by in the drawing, the detailed operation of thev several modifications shown will now be explained.

It is well known that a neon tube can be made to oscillate when connected in series with a, suit: able battery and a resistance of the proper value. The frequency of oscillation may be varied by changing the resistance or by shunting the tube with a variable condenser. The action depends upon the fact that at low voltages no current will flow through the valve, but when the potential rises to a certain point the gas ionizes and the resistance in the tube drops to a very low value. The tube will glow and continue to draw current unless the potential impressed on the tube is considerably reduced. As soon as the current starts to flow, the potential drop across the resistance in series with the tube, reduces the tube potential to a sufficiently low value to stop the ionization and cut oil the current. The tube potential then builds up again until the tube again discharges, and the cycle of operation is repeated. The higher the resistance and the higher the capacitance across the tube, the longer it takes to build up to an ionization potential, and hence the lower the frequency of oscillation.

The carrier current oscillator shown in Fig. 1 may be considered as two separate oscillating circuits operating on the above principle. Neon tube 5 is connected in series with resistances 9 and 12, the left half of the battery 11, and the v induction coil 13, and is shunted by the variable condenser 7. The neon tube 6 is connected in 1 series with the resistances 10 and 12, the right half of the battery 11, and the induction coil 13, and is shunted by the variable condenser '8. The common branch of the two circuits comprising 7 resistance 12 and coil 13 is connected to the middle point of the battery 11 so that the pulsations of the two oscillating circuits pass through the If the resistance in the common branch of the two oscillating circuits is small, the circuits have practically no effect on each other, but if this resistance is made large they tend to synchronize very readily when adjusted to nearly the same frequency. The frequency of oscillation of each circuit may be adjusted by the variable condensers 7 and 8, respectively. When the two circuits have once been synchronized, the adjustment of one or both may be changed con-- siderably without throwing the two out of synchronism.

This synchronizing of the two oscillating circuits will now be explained with reference to the oscillator shown at the left of Fig. 1. It will be assumed that the potential across the neon tube 6 has built up to nearly the ionization potential of the tube at the time the two osci1lating circuits are coupled together. When the tube 6 strikes the sudden surge of current through the coupling resistance 12 causes an appreciable RI drop across this resistance. This RI drop is in a direction so it opposes the E. M. F. of the right half of the battery 11 which is producing the current. It will be readily seen that when the RI drop across the resistance 12 opposes the E. M. F. of the right half of battery 11, it will assist and add to the E. M. F. of the left half of battery 11 when considering the circuit of the neon tube 5. This increase in potential in the circuit of the tube 5 causes the potential across the tube to quickly reach the ionization potential and the tube strikes soon after the surge of current through the tube 6. When the tube 5 strikes the surge of current through the coupling resistance 12 is now in the opposite direction and the RI drop across the resistance now adds to the potential of the right half of the battery 11 and thereby causes the tube 6 to again strike. It is, therefore, readily seen that the increase in potential across one tube when the other strikes causes the two neon tubes to pull together and will keep them oscillating at the same frequency. It is apparent that the larger the coupling resistance between the two circuits, the greater the RI drop across the resistance when one tube strikes and the .greater the increase in potential across the other tube.

From the foregoing it is apparent that whenever the tubes 5 and 6 are out of step the current through the common branch of the oscillator is first a surge in one direction and then a surge in the other. The wave form of the resultant current, therefore, approaches a sine wave but half waves are sharper and of somewhat saw tooth form. Considering the resultant current as regarded from the output circuit, the two currents may be as much as 180 degrees out of phase and the resultant is an alternating current of approximately sine Wave form. The output current is a maximum when the circuits are oscillating in this manner. With the two tubes 5 and 6 striking in phase, the output will be zero.

If one of the circuits is adjusted so that it would oscillate at a higher frequency if it were alone, it will tend to oscillate at this frequency when the two are coupled and will pull the oscillations of the other circuit with it, the result being that they will oscillate at some intermediate frequency. In this case one of the circuits is tending to oscillate faster and. the other is tending to oscillate slower with the result that one has a leading phase and the other a lagging phase, consequently the resultant output current is re- ,duced.

Referring now to Fig. l, the two circuits are adjusted to operate at the same frequency and are then coupled as shown in the drawing. The variable condenser in parallel with one of the neon tubes is then adjusted to increase its capacity while the variable condenser in parallel with the other tube is adjusted to decrease its capacity an equal amount so that the frequency at which both circuits are oscillating will remain the same, but an output current will be produced due to the difference in phase between the two currents. The oscillator may be normally adjusted so that the resultant output current is about one-half of its maximum value. The telephone transmitter 15 is connected in series with the battery 46 and the primary winding 14 of an iron core transformer. The secondary winding 13 of this transformer is connected in series with the resistance 12 in the common branch of the two oscillating circuits.

As stated above, it will be assumed that the oscillator is normally operating at the carrier current frequency with the phase relation between the two oscillating circuits so adjusted that the output current is about one-half of the maximum value. The grid of the first amplifying tube is connected to some point of the resistance 12 with a variable connection so that the voltage on the grid of the tube may be readily adjusted to an operative value. The output current flowing in the common branch through the resistance 12 and the coil 13 sets up a fluctuating potential on the grid of the amplifying tube 16 and changes its potential with respect to the filament of the tube by an amount equal to the RI drop across the resistance 18. This varying potential on the grid of tube 16 results in a corresponding plate current through the resistance coupling 19 to the positive pole of battery 11. The varying RI drop across resistance 19 produces a corresponding potential variation on the grid of tube 23 with respect to the filament of this tube. The resistance 22 connected between the grid and the filament of the tube 23 provides a normal biasing potential on the grid of the tube.

As a result of the varying potential on the grid of tube 23 a plate current flows through the primary winding 25 of the transformer to the positive pole of the battery 11. This current induces a corresponding current in the secondary winding 26 which is connected in series with the primary windings 27 and 28 of the coupling transformers. This current is induced in the secondary windings 29, 31, and 30, 32 of the hybrid coil and is superimposed on the main telephone line comprising conductors 1 and 2 through the condensers 3 and 4, respectively. Thus a carrier current of the frequency to which the oscillator is adjusted is normally being superimposed on the telephone line.

Referring now to the transmitter circuit, a small direct current is normally flowing through the transmitter 15 and the primary winding 14 of the iron core coupling transformer. Assuming that conversation is taking place, the resistance of the transmitter is varied in accordance, with the sound waves of the speech and these changes in resistance cause corresponding changes in the current flowing through the winding 14 of the transformer. The changes in current through the primary winding 14 induce E. M. F.s in the secondary winding 13 which is included in the common branch of the differential oscillator. It will be seen that any E. M. F. induced in the coil 13 will effect the two oscillating circuits in the opposite manner. For example, if, at a certain instance, the induced E. M. F. is in such a direction so as to assist the left half of the battery 11 it will tend to make the tube 5 oscillate at a higher frequency but at the same time the induced E. M. F. opposes the right half of the battery 11 and tends to slow down the frequency of oscillation of the tube 6. The result is that the two oscillating circuits continue to operate at the same frequency but the phase relation between the two pulsating currents will be shifted thereby causing an increase or a decrease in the magnitude of the output current depending on the direction of the induced E. M. F. and the manner in which the phases were shifted to produce the normal output current. Since the normal output current was about half of the maximum value, the modulation of the carrier by voice currents may so shift the phase relation between the two pulsating currents that the resultant output current may Vary from zero to full maximum value without any change in frequency. The modulated current is amplified by the two resistance coupled amplifiers 16 and 23 and .is superimposed on the telephone line through the coupling transformers and the two condensers 3 and 4.

Having described the modulation of the carrier current by the voice on an outgoing conversation,

.the operation of the receiving circuit will now be explained. The incoming carrier current wave which has been modulated in accordance with speech currents at some other station, passes through condensers 3 and 4, windings 29 and 31 of the hybrid coil, and through the primary winding 34 of the receiver transformer. The secondary winding 35 of this transformer is connected to the grid of the rectifying tube 36, through the usual grid leak and condenser 38 and 37, respec- .tively. The modulated carrier current is rectified phone transmitter 15 and its coupling transformer have been replaced by a diiferential condenser microphone 61. The center or vibrating element of the microphone is connected to the common branch of the oscillator. The two fixed plates of the microphone are connected to the upper terminals of the two neon tubes, respectively. Thus each section of the microphone is bridged across one of the neon tubes. When the center element or plate of the microphone vibrates in accordance with sound waves it varies the capacity of each section of the microphone, increasing the capac ity of one section while decreasing the capacity of the other section. Thus the capacity bridged across one neon tube is slightly increased while that bridged across the other neon tube is slightly decreased thereby shifting the phase relation between the two currents and modulating the resultant output current in accordance with the voice frequencies. The conductors 58, 59, and 60 correspond to conductors 42, 43, and 44, respectively, of Fig. 1. The remainder of the circuit in a system using a condenser microphone would be the same as the remainder of the circuits shown in Fig. 1.

Referring now to Fig. 3 of the drawing, the

invention has been shown as applied to a carrier current telegraph system. One telegraph station is shown in Fig. 3 and is coupled to themain telegraph line comprising conductors 101 and 102 by means of condensers 103 and 104 and the usual balanced coil and artificial line arrangement in the same manner as described above for the carrier current telephone system. The operation of the differential neon tube oscillator is the same as previously described. In this case an additional neon tube oscillator adjusted to operate at an audible frequency is provided. This oscillator circuit comprises the neon tube 152 with the vari able condenser 153 in parallel, a battery 154, a high resistance 151, and the primary winding 114 of an iron core transformer. The telegraph key 150 is connected so that it normally short-circuits the primary winding 114 of the transformer.

When the telegraph key is operated in accordance with the telegraph signals, the primary winding 114 of the transformer is intermittently included in the circuit of the oscillator and a current of audible frequency is allowed to pass through the coil each time the telegraph key is opened. The induced E. M. F. in the secondary winding 113 due to this current modulates the output carrier current of the differential neon tube oscillator in accordance with the signals of the telegraph message. This modulation is effected by the shifting of the phase relation between the two oscillating currents as described above in connection with the operation of Fig. 1. The modulated carrier current is then amplified by the two resistance coupled amplifiers 116 and 123 and superimposed on the telegraph line through the transformers and the condensers 103 and 104.

At the receiving station the incoming carrier current passes through condensers corresponding to condensers 103 and 104, through transformer windings corresponding to windings 129 and 131 and through the primary winding of a receiving transformer corresponding to winding 134. It will be assumed that the station shown in Fig. 3 is the receiving station. The incoming modulated carrier current passes through the primary winding 134 of the transformer and induces a coresponding E.M.F.in the secondary winding 135. The secondary winding is connected to the grid of the rectifying tube 136 through the grid leak and condenser 138 and 137 in the usual manner. The current is rectified by the tube 136 and current will intermittently flow in the plate circuit of the tube in accordance with the telegraph signals sent out at the transmitting station. This plate current intermittently operates the tuned reed relay 141. The reed 155 of this relay is tuned so that its natural period of vibration corresponds to the frequency to which the neon tube oscillator, used to modulate the carrier current, is adjusted. Each time the telegraph key at the transmitting station is opened to allow current to flow through the primary winding of the coupling transformer, a plate current is caused to flow through the tuned reed relay 141 at the receiving station. The relay 141 responds to this plate current and the vibrations of the tuned reed 155 cause the spring 156 to be operated and to break its contact with spring 157. The opening of these contacts opens the circuit of relay 145 which accordingly releases and closes its contact 149. The closing of contact 149 completes the circuit for the telegraph sounder 146. Accordingly the telegraph sounder is intermittently operated in accordance with the dots and dashes of the telegraph message sent out from the transmitting station. The filament battery 139 of the rectifying tube is used for supplying battery to the relay 145 and the telegraph sounder 146.

' Although only a single channel has been illustrated in Fig. 3, it will be apparent that a num ber ofchannels could be provided by providing a number of differential neon tube oscillators each tuned to a different frequency and a corresponding number of filters for the receiving circuits. The invention has been illustrated. as applied to a carrier current telephone and a carrier current telegraph system, but it will be apparent to those skilled in the art that the novel form of oscillator and the method of modulating its output current may be applied to many uses other than those shown and described in this specification.

Having described the invention, what is thought to be new and is desired to have protected by Letters Patent will be pointed out in the appended claims.

What is claimed is:

1. In combination, a differential oscillator comprising two neon tube oscillating circuits coupled by a common resistance, and means for modulating the output of said oscillator by shifting the phase relation between the currents in said two oscillating circuits.

2. In combination, two neon tube oscillating circuits having a common branch, a resistance in said common branch for synchronizing said..cir-

cuits, and means for modulating the resultant output current of said circuits by shifting the phase relation between the currents in said two circuits while keeping them synchronized.

3. In combination, two neon tube oscillating circuits having a common branch, a resistance in said branch common to said circuits, circuit arrangements such that the pulsating currents of said two circuits pass through said resistance in opposite directions and are synchronized at the same frequency, and means inductively coupled to said common branch for modulating the resultant output current of said circuits.

4. In combination, a differential oscillator comprising two neon tube oscillating circuits having a common branch, each circuit consisting of a neon tube with a variable condenser in parallel, a resistance, and a battery connected in series with said common branch, a resistance in said common branch for synchronizing said two oscillating circuits, and'a transmitter inductively coupled to said common branch for modulating the resultant output current of said oscillator in accordance with voice currents.

5. In combination, two oscillator circuits having a common branch, a resistance in said common branch for coupling said circuits and synchronizing the currents in said two circuits at the same frequency, a common output circuit, and means for modulating the resultant current in said output circuit by shifting the phase relation between the currents in said two oscillator circuits while keeping them synchronized at the same frequency.

6. In a carrier current telephone system, a line, a plurality of stations connected to said line, a transmitter at each of said stations comprising a differential neon tube oscillator having two oscillating circuits, means for modulating the output current of said oscillator in accordance with speech currents by shifting the phase relation between the currents in said branches, and means at each of said stations responsive to said modulated current for reproducing the speech.

'7. In a carrier current transmission system, a

line, a plurality of stations connected to said line,-

means at each station for generating a carrier current of a definite frequency, said means including an oscillator having two neon tube oscillating circuits, means for modulating said carrier current in accordance with signals by shifting the phase relation between the currents in said two circuits without changing the frequency, and means at each station responsive to said modulated current for reproducing said signals.

8. In combination, two pulse generating circuits having a common branch including a resistance, circuit connections such that the direct current pulses from said two circuits pass through said resistance in opposite directions and are synchronized at the same frequency, and means for modulating the resultant alternating current by shifting the phase relation between the two pulsating currents.

9. In combination, a differential oscillator comprising two neon tube oscillating circuits having a common branch, a resistance in said common branch for synchronizing said two circuits to operate at the same frequency, and a differential condenser microphone for modulating the resultant output current in accordance with voice currents, said microphone having its two condenser elements connected in multiple with said two tubes, respectively.

10. In combination, a differential oscillator comprising two neon tube oscillating circuits having a common branch, a resistance in said common branch for synchronizing said circuits to operate at the-same frequency, a second oscillator operating at a diiferent frequency, and means for inductively coupling said second oscillator to said common branch at will to modulate the resultant output current of said differential oscillator in accordance with signals.

LAURENCE J. LESH. 

